Orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA) systems use not a single-carrier modulation technique but a multi-carrier modulation technique that transmits data in parallel using a plurality of sub-carriers having mutual orthogonality. According to the OFDMA technique, OFDMA symbols are respectively bundled and transmitted over pre-determined sub-channels by sub-carriers, thereby transmitting data.
In a wireless communication system, transmitted signals are generally affected by multi-paths having different amplitudes and delays. Due to such multi-paths, fading occurs and thus signals received by a receiver are distorted. In order to compensate for distortion of received signals caused by the above-mentioned problem and to ensure mobility of communication terminals, means for estimating a channel estimation value and equalizing a received signal according to the channel estimation value is in demand.
As is well known to those skilled in the art, in order to estimate a channel of a signal transmitted and received in a wireless communication system, a training symbol previously defined between a transmitter and a receiver is necessary. In particular, in a noisy environment causing multi-echoes to a signal transmitted through an air channel, channel estimation is performed to equalize a received signal and accurately decode the received signal using pilot symbols included in the signal. Such pilot symbols are transmitted within all OFDMA symbols of an uplink frame.
FIG. 1 illustrates an example of an OFDM time division duplex (TDD) frame structure based on the wireless broadband Internet (WiBro) standards to which the present invention will be applied. The frame structure illustrated in FIG. 1 conforms to the Institute of Electrical and Electronics Engineers (IEEE) 802.16d/e standards.
Referring to FIG. 1, a transmission (Tx)/reception (Rx) transition gap (TTG) is inserted between a downlink and an uplink, and an Rx/Tx transition gap is inserted between the end of a frame and the start of another frame. In OFDMA systems, data is transmitted in an uplink through sub-channels respectively allocated to subscribers. Such an uplink frame may be composed of a plurality of zones. The respective zones are classified according to an OFDMA sub-channel allocation scheme, and may vary according to each OFDMA symbol. The uplink sub-channel allocation scheme uses channel modes such as uplink (UL)-partial usage of sub-channels (PUSC), UL-optional PUSC (OPUSC), UL band-adaptive modulation and coding (AMC), etc. A ranging sub-channel illustrated in the lower end of the uplink frame illustrated in FIG. 1 is used for uplink synchronization and power control between a mobile terminal and a base station, bandwidth request of a mobile terminal, and so on. The WiBro standards define 4 modes which are an initial ranging mode, a periodic ranging mode, a hand off ranging mode, and a bandwidth ranging mode.
In an uplink, synchronization is performed by the above-mentioned ranging process. In such an uplink, since channel environments of mobile terminals are different from each other, the point of a signal receiving time may vary and the amount of received power also may vary. A base station receives uplink signals of several mobile terminals passed through different channel environments and thus must estimate a channel of each user.
This specification improves a method of estimating a channel for such an uplink signal and suggests a channel estimation method and device for enhancing accuracy of channel estimation in a time-variant environment as well as a time-invariant environment.